1. FIELD OF THE INVENTION
The present invention relates generally to the field of circulating fluidized bed boilers and, in particular, to a new and useful configuration for reducing or eliminating tube erosion in the region of the top of the refractory covering on lower furnace walls, or on wing walls or division walls.
2. BACKGROUND OF THE INVENTION
In circulating fluidized bed boilers, the problem of erosion of tubes at the top edge of refractory lining is well known.
In a circulating fluidized bed boiler, reacting and non-reacting solids are entrained within the enclosure by the upward flow of gases which carry some solids to the reactor exit at the upper end of the reactor. Other, larger quantities of solids are recycled within the reactor enclosure as heavier solids initially carried upwards fall back against the flow of gases. Since the velocity of the upward flow of gases is often much lower in the cooler gases adjacent the circulating fluidized bed enclosure walls and heat transfer surfaces within the circulating fluidized bed, most of the solids fall near the walls or heat transfer surfaces.
The amount of solids falling adjacent to the walls and surfaces increases progressively toward the bottom of the circulating fluidized bed. The density of the bed is higher in the lower regions of the furnace, and as a result, the walls and surfaces in the lower regions are subject to increased erosion from contact with the solids.
Further, the reactions occurring in the circulating fluidized bed create chemical reduction conditions against which the walls and heat transfer surfaces must be protected. A protective material (further called refractory) is often used to coat the walls and exposed surfaces in the lower regions of the circulating fluidized bed. The refractory material, anchoring and installation is expensive, since it must withstand high temperatures (typically between 1400.degree. and 1800.degree. F.), contact erosion from solids, and chemical reduction and by-products from the combustor reactions. The refractory also reduces the efficiency of the heat transfer. For this reason, refractory is only applied to the walls and exposed surfaces to as low an elevation in the reactor region as possible considering corrosion and erosion conditions. At the point on the walls and surfaces where the refractory terminates, a discontinuity is formed where erosion of the metal of the tubes forming the walls occurs. The erosion is typically in a band about 1/4" to 3" wide adjacent the top edge of the protective material. Tube wall erosion is found in an area between 0 and 36 inches above the top of the refractory.
One method for reducing this erosion is found in U.S. Pat. No. 5,893,340 to Belin et al. in which the walls of the enclosure are bent into and out of the solid flow stream to reduce the incidence of solids on the refractory discontinuity.
An alternative known method is to place a protective overlay material on the tube at the refractory discontinuity as a shield. The protective overlay extends from below the termination of the refractory to several inches above the discontinuity. Unfortunately, the protective overlay suffers the same erosion and must eventually be replaced in an expensive and time consuming procedure.
None of the prior methods are completely successful in eliminating erosion near the refractory.